The invention relates generally to the detection of cardiac activity in a patient, and more specifically, to the detection of a cardiac pulse.
The presence of cardiac pulse, or heartbeat, in a patient is often detected by palpating the patient""s neck and sensing changes in the volume of the patient""s carotid artery due to blood pumped from the patient""s heart. A graph representative of the physical expansion and contraction of a patient""s carotid artery during two consecutive heartbeats is shown at the top of FIG. 1. When the heart""s ventricles contract during a heartbeat, a low-frequency pressure wave is sent throughout the patient""s peripheral circulation system. The carotid pulse shown in FIG. 1 rises with the ventricular ejection of blood at systole and peaks when the ejection rate is at its maximum. The carotid pulse falls off again as the pressure subsides toward the end of each pulse.
The opening and closing of the patient""s heart valves during a heartbeat causes high-frequency vibrations in the adjacent heart wall and blood vessels. These vibrations can be heard in the patient""s body as heart sounds. A conventional phonocardiogram (PCG) transducer placed on a patient converts the acoustical energy of the heart sounds to electrical energy, resulting in a PCG waveform that may be recorded and displayed, as shown by the middle graph in FIG. 1. Conventional methods for detecting and displaying a PCG waveform are well-known in the art. See, e.g., U.S. Pat. Nos. 5,687,738 and 4,548,204.
As indicated by the PCG waveform shown in FIG. 1, a typical heartbeat produces two main heart sounds. The first heart sound, denoted S1, is generated by vibration generally associated with the closure of the tricuspid and mitral valves at the beginning of systole. Typically, the heart sound S1 is about 14 milliseconds long and contains frequencies up to approximately 500 Hz. The second heart sound, denoted S2, is generally associated with vibrations resulting from the closure of the aortic and pulmonary valves at the end of systole. While the duration of the second heart sound S2 is typically shorter than the first heart sound S1, the spectral bandwidth of the heart sound S2 is typically larger than that of S1.
An electrocardiogram (ECG) waveform of a patient is different than a PCG waveform. An ECG waveform describes the electrical activity, rather than the acoustical activity, of a patient""s heart. The bottom graph of FIG. 1 illustrates an example of an ECG waveform corresponding in time with the carotid pulse and the PCG waveform.
The lack of a detectable cardiac pulse in a patient can be a strong indicator of cardiac arrest. Cardiac arrest is a life-threatening medical condition in which the electrical activity of a patient""s heart becomes unsynchronized, resulting in a loss of the heart""s ability to contract and pump blood into the circulation system. A caregiver may apply a defibrillation shock to a patient in cardiac arrest to stop the unsynchronized electrical activity and reinitiate a synchronized perfusing rhythm. External defibrillation, in particular, is provided by applying a strong electric pulse to the patient""s heart through electrodes placed on the surface of the patient""s body.
Before providing defibrillation therapy to a patient, a caregiver must first confirm that the patient is in cardiac arrest. A defibrillation shock provided to a patient not in cardiac arrest may itself induce lethal cardiac arrhythmias in the patient. In general, external defibrillation is suitable only for patients that are unconscious, apneic (i.e., not breathing), and pulseless. Medical guidelines indicate that the absence of a pulse in a patient should be determined within 5-10 seconds.
Unfortunately, under the pressures of an emergency situation, it can be extremely difficult for first-responding caregivers with little or no medical training to consistently and accurately detect a cardiac pulse in a patient (e.g., by palpating the carotid arteries) in a short amount of time such as 5-10 seconds. Nevertheless, because time is of the essence in treating cardiac arrest, a caregiver may rush the preliminary evaluation, incorrectly conclude that the patient has no pulse, and proceed to provide defibrillation therapy when in fact the patient has a pulse. Alternatively, a caregiver may incorrectly conclude that the patient has a pulse and erroneously withhold defibrillation therapy. A need therefore exists for a method and apparatus that quickly, accurately, and automatically determines the presence of a pulse in a patient, particularly to assist a caregiver in determining whether defibrillation therapy is appropriate in an emergency situation.
The present invention provides a method and apparatus that determines the presence of a cardiac pulse in a patient by evaluating the patient for the presence of characteristic heart sounds. By physiologically associating the presence of a heart sound with the presence of a cardiac pulse, the presence of a cardiac pulse in the patient is determined.
In accordance with one aspect of the present invention, the presence of a cardiac pulse in a patient is determined by analyzing PCG data obtained from the patient for a feature indicative of the presence of a heart sound and determining from the feature whether a heart sound is present in the patient. Analyzing the PCG data includes evaluating the temporal energy in the PCG data or evaluating the spectral energy in the PCG data. Evaluating the temporal energy in the PCG data may include estimating an instantaneous energy in the PCG data, estimating a background energy in the PCG data, and comparing the estimated instantaneous energy with the estimated background energy. Evaluating the spectral energy in the PCG data may include calculating an energy spectrum of the PCG data, evaluating the energy spectrum to locate a peak energy value, and comparing the peak energy value with a threshold energy value. The frequency at which a peak energy value occurs may also be compared with a threshold frequency. The peak energy value used in this aspect of the invention is preferably the second peak energy value occurring in the energy spectrum measured from DC.
In accordance with another aspect of the present invention, the presence of a cardiac pulse in a patient may be determined by combining an evaluation of the temporal energy in the PCG data with an evaluation of the spectral energy in the PCG data. Electrocardiogram (ECG) data obtained from the patient may also be used in connection with the PCG data to determine the presence of a pulse in the patient. In one approach, if the PCG data appears to indicate the presence of a heart sound, the ECG data is evaluated for the presence of a QRS complex. If the time at which the QRS complex occurs is within an expected time of when the heart sound appeared to occur, a cardiac pulse is determined to be present in the patient. In another approach, the ECG data is evaluated for the presence of a QRS complex to gate the heart sound detection process. If an R-wave occurs in the ECG data, the PCG data should indicate the presence of a heart sound following the R-wave if a cardiac pulse is present. If a heart sound is not detected following an R-wave, the patient may be in a state of pulseless electrical activity (PEA).
In yet another aspect, the present invention provides a medical device with at least one electrode adapted to sense PCG signals in a patient, a conversion circuit for converting the PCG signals into digital PCG data, and a processing unit for processing the PCG data to determine a feature indicative of the presence of a heart sound in the patient. The processing unit determines the presence of a heart sound in the patient based on the determined feature. The medical device may further include a defibrillation pulse generator for providing a defibrillation pulse to the patient, and an input device that allows the operator of the device to initiate delivery of the defibrillation pulse, if defibrillation therapy is appropriate.
The medical device may determine the presence of a heart sound in the patient by evaluating the temporal energy in the PCG data or the spectral energy in the PCG data, or both. The medical device may also be provided with ECG electrodes adapted to sense ECG signals in the patient. The processing unit evaluates ECG data in combination with the PCG data to determine whether a cardiac pulse is present in the patient. A display is included for prompting messages to the operator of the device. When implemented in an automated external defibrillator, physiological data, such as PCG data, obtained from the patient is automatically evaluated for the presence of a cardiac pulse to assist in determining whether a defibrillation pulse should be applied to the patient.